It is well known in the transmission of radar waves for instance, to use cavity waveguides which are provided with slots from which the electromagnetic field radiates. It is desirable in this regard to be able to choose the polarization of the radiated field. U.S. Pat. No. 2,982,960 teaches an antenna having hollow waveguides which are capable of emitting such a field of desired polarization. The antenna has a waveguide which is fed by two mutually perpendicular inputs on the sides of the waveguide. Two orthogonal fields are excited in the waveguide via the inputs. The waveguide is provided on one side thereof with mutually intersecting and transversely and longitudinally extending slots, each of which radiates a respective one of the aforesaid two orthogonal fields. However, the antenna has the drawback of producing higher-order radiation loads, so-called grating lobes, if the slots are located at a resonant distance from one another. When there is no resonant distance between the slots, there is obtained an antenna lobe which radiates laterally from the geometric normal of the antenna and the direction of which is frequency-dependent.
U.S. Pat. No. 3,348,227 teaches an antenna having a cavity waveguide which is provided on its broadest side with mutually separated and transversely and longitudinally extending slots. Energy is delivered to the antenna in an oscillating mode and the antenna radiates a field whose direction of polarization can be chosen in accordance with the way in which the energy is delivered. Energy is delivered to the slots through the common waveguide and only one polarization direction can be chosen at any one moment in time. Consequently, only one information-carrying signal can be transmitted.
An overall view of individual antennas provided with slotted waveguides and providing selective polarization is given in an article in MIKROWELLEN & HF MAGAZIN, Vol. 15, No. 3, 1989, by A. J. Sangster: "Polarisation Diversity Techniques for Slotted-Wave-guide Antennas". Of the antennas illustrated, the antenna which is most relevant in the present case is the antenna shown in FIG. 9 of the article, which illustrates a waveguide having longitudinal and transversal slots. The waveguide is intended for higher-order propagation modes of an electromagnetic wave and one drawback with this particular waveguide resides in its large width. Waves emanating from several mutually adjacent waveguides are able to generate an antenna lobe which can be directed laterally by phase-shifting the waves supplied to the different waveguides. However, this results in grating lobes in the lateral direction. Grating lobes also occur in the longitudinal direction, since the slots are placed apart at a resonant distance along the waveguide. When the slots are placed closer together, the grating lobes are avoided in this latter direction, although the radiation lobe is directed obliquely, the direction of said lobe being frequency-dependent.